With the advent of solid state control logic means such as microcomputers or microprocessors, a whole new field of control devices has evolved. When these devices are used in condition control or process control applications, the solid state control logic means or microcomputer ultimately controls heavy duty electrical switching equipment, such as relays. While the microcomputer or microprocessor operations entail possible failure modes that must be guarded against, they also provide an almost unlimited ability to monitor and control related equipment in fail safe manners not previously available in the control art. The ability of the microprocessor or microcomputer to carry out a large number of control functions in an exceedingly short period of time makes this type of a device an ideal tool for monitoring and control of associated equipment.
In order to provide a degree of safety that is comparable with electromechanical types of devices, microprocessor or microcomputer type condition control systems must be operated with control routines that are significantly different than the mode of control applied to electromechanical types of units. These routines form types of safety checking modes for the device.